Why don't they shoot him in the leg?
Shooting someone in the leg is risky. You have a smaller target (so it is easier to miss and hit something you aren't aiming for, like a bystander or another cop), you can hit a major artery (so they'll bleed out and die anyways), and even if you do hit him, there is no guarantee that shooting him in the leg will cause him to cease being a threat.

Cops can do whatever they want, we can't trust them!
Police are not allowed to run roughshod over the citizenry. If one of them steals a candy bar, it is theft, and if caught they will lose their career over it. If one of them has legitimately used excessive force, they are accountable to not only their department, but to the legal system. If an officer arrests you without probable cause, or searches you without sufficient justification, it can compromise an entire case and allow a horrible person out on the streets because they didn't bother to follow procedure. Sure, a few get away with doing bad things, but a few criminals also get away with doing bad things. The vast majority of officers are accountable for their actions.

I got arrested for fighting and the cop told me I should make better decisions and I was so pissed, you know?
So the officer gives them a very good suggestion as to how to not be arrested anymore, and they acted like a petulant child?

LEOs CAN be wrong. They frequently are-- they are human, after all. These aren't "differing opinions" they are arguments born out of ignorance or a grudge against law enforcement.

Another argument about the homeless man shot in LA--

"Why didn't they just slap some handcuffs on him?"
He was very clearly resisting arrest and doing quite a good job of fending off three officers. He wasn't standing still long enough to get a handcuff on him in the first place. Cuffs aren't magical metal shackles that attach to people through willpower, they have to be applied by an officer and typically the suspect's arm needs to be still for a few seconds to allow the officer to secure the cuffs on him.

Bottom line: LEOs have a tough job, and sitting on the sidelines, judging every action they take and then informing the officers what they should have done in the situation is just irritating. You can always sit back and analyze something after the fact, scrutinizing it for months until you decide what course of action the officer should have taken-- the officer had a quarter second to decide. He shoots-- excessive force, the suspect was unarmed and he could have shot him in the leg or used his taser! He doesn't shoot-- officer down, the suspect has escaped, he's killed some kid while fleeing the scene, why aren't the police doing their jobs?